1. Field of the Invention
The present invention relates to a method of determining the density of toner in developer used in a developing unit of an image forming apparatus, and particularly relates to a method of determining the density of toner in the developer by the use of a density sensor located in the vicinity of a stirring member which is disposed in a developer tank so as to stir the developer contained therein.
2. Description of the Prior Art
An electrophotographic image forming apparatus generally uses developer composed of toner and carrier for the formation of images, the toner being made of a resin and a colorant, and the carrier being made of powdered iron or the like. In an image forming process, the toner in the developer contained in a developer tank is consumed in developing a latent image formed on a photoconductor. Thus., as the image forming process is repeated, the density of toner in the developer (the weight ratio of toner to the developer) contained in the developer tank decreases. When the developer with reduced toner density is further used for image forming processes, fogging may occur in the resultant images, or the surface of the photoconductor may be scratched by the carrier in the developer. Thus, toner should be fed into the developer tank as the density of toner in the developer is decreased.
An image forming apparatus such as described above is provided with a density sensor for measuring the toner density in the developer, so that toner is supplied from a toner hopper into the developer tank when the measured value is less than a predetermined reference value. A sensor which measures magnetic permeability is usually employed as the density sensor.
In a conventional method of determining the density of toner in developer, the density sensor is installed in a position facing a developing roller disposed in the developer tank, or in the vicinity of a stirring member in the form of a screw or a blade which is also disposed in the developer tank so as to stir the developer.
When the density sensor is installed in a position facing the developing roller, however, the position of the sensor is limited to that above or below the developing roller because the photoconductor and the stirring member are disposed at both sides of the developing roller in a horizontal direction. The disposition of the density sensor above or below the developing roller increases the height of the developer tank having the developing roller and the stirring member disposed therein. The developer tank is so located as to face the photoconductor. Since many other process units must be located facing the photoconductor, the increase in the height of the developer tank is not desirable.
On the other hand, when the density sensor is installed in the vicinity of the stirring member, the developing roller, the stirring member and the density sensor can be arranged in that order substantially in a horizontal plane, so that the height of the developer tank can be kept small. With this arrangement, however, the toner density cannot be accurately determined. Since the stirring member rotates to stir the developer in the developing tank to mix the toner and the carrier, the density of the toner in the developer in the area near the stirring member fluctuates with the rotation of the stirring member. Thus, the value to be obtained by the density sensor fluctuates as the stirring member makes a revolution. In the conventional method, the density of toner is determined by the value obtained at a certain point of time. Thus, the determined value of the toner density is inaccurate because of the above-mentioned density fluctuation with the rotation of the stirring member. Furthermore, when the flowability of the developer decreases due to external disturbance such as changes in environment, the fluctuation in the toner density is increased, which further lowers the accuracy in the determination of the toner density.